lseek - reposition read/write file offset
off_t lseek(int fd, off_t offset, int whence);
The lseek() function repositions the offset of the open file associated
with the file descriptor fd to the argument offset according to the
directive whence as follows:
The offset is set to offset bytes.
The offset is set to its current location plus offset bytes.
The offset is set to the size of the file plus offset bytes.
The lseek() function allows the file offset to be set beyond the end of
the file (but this does not change the size of the file). If data is
later written at this point, subsequent reads of the data in the gap (a
"hole") return null bytes (' ') until data is actually written into
Seeking file data and holes
Since version 3.1, Linux supports the following additional values for
Adjust the file offset to the next location in the file greater
than or equal to offset containing data. If offset points to
data, then the file offset is set to offset.
Adjust the file offset to the next hole in the file greater than
or equal to offset. If offset points into the middle of a hole,
then the file offset is set to offset. If there is no hole past
offset, then the file offset is adjusted to the end of the file
(i.e., there is an implicit hole at the end of any file).
In both of the above cases, lseek() fails if offset points past the end
of the file.
These operations allow applications to map holes in a sparsely
allocated file. This can be useful for applications such as file
backup tools, which can save space when creating backups and preserve
holes, if they have a mechanism for discovering holes.
For the purposes of these operations, a hole is a sequence of zeros
that (normally) has not been allocated in the underlying file storage.
However, a filesystem is not obliged to report holes, so these
operations are not a guaranteed mechanism for mapping the storage space
actually allocated to a file. (Furthermore, a sequence of zeros that
actually has been written to the underlying storage may not be reported
as a hole.) In the simplest implementation, a filesystem can support
the operations by making SEEK_HOLE always return the offset of the end
of the file, and making SEEK_DATA always return offset (i.e., even if
the location referred to by offset is a hole, it can be considered to
consist of data that is a sequence of zeros).
The _GNU_SOURCE feature test macro must be defined in order to obtain
the definitions of SEEK_DATA and SEEK_HOLE from <unistd.h>.
The SEEK_HOLE and SEEK_DATA operations are supported for the following
* Btrfs (since Linux 3.1)
* OCFS (since Linux 3.2)
* XFS (since Linux 3.5)
* ext4 (since Linux 3.8)
* tmpfs (since Linux 3.8)
Upon successful completion, lseek() returns the resulting offset
location as measured in bytes from the beginning of the file. On
error, the value (off_t) -1 is returned and errno is set to indicate
EBADF fd is not an open file descriptor.
EINVAL whence is not valid. Or: the resulting file offset would be
negative, or beyond the end of a seekable device.
The resulting file offset cannot be represented in an off_t.
ESPIPE fd is associated with a pipe, socket, or FIFO.
ENXIO whence is SEEK_DATA or SEEK_HOLE, and the current file offset is
beyond the end of the file.
SVr4, 4.3BSD, POSIX.1-2001.
SEEK_DATA and SEEK_HOLE are nonstandard extensions also present in
Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion in
the next POSIX revision (Issue 8).
Some devices are incapable of seeking and POSIX does not specify which
devices must support lseek().
On Linux, using lseek() on a terminal device returns ESPIPE.
When converting old code, substitute values for whence with the
Note that file descriptors created by dup(2) or fork(2) share the
current file position pointer, so seeking on such files may be subject
to race conditions.
dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)
This page is part of release 3.65 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.